1. Technical Field
The present disclosure relates to a broadcast signal demodulation apparatus and method, and more particularly, to a quadrature phase shift keying demodulator of digital broadcast reception system and demodulation method thereof.
2. Description of the Related Art
Korean Patent Publication No. 2002-95741 relates to a technique of a Quadrature Phase Shift Keying (QPSK) demodulator for an open cable standard. Korean Patent Publication No. 1999-54384 relates to a QPSK demodulation technique where the time taken for a frequency offset to converge to 0 (zero) is reduced.
Digital broadcast transmission may utilize a Vestigial Side-Band (VSB) modulation method, a Quadrature Amplitude Modulation (QAM) method, or a QPSK modulation method, etc.
FIG. 1 illustrates frequency bands of a QAM modulated signal and a QPSK modulated signal in a single coaxial cable.
As illustrated in FIGS. 1, a 70 to 130 MHz frequency band is assigned to transmit digital broadcast data using a coaxial cable according to the QPSK modulation method. In addition, a QPSK demodulator is required to receive digital broadcast data in the 70 to 130 MHz frequency band. The QPSK modulation method is selected as a data transmission standard of the open cable standard.
According to the open cable standard, the QPSK demodulator demodulates data using an imaginary noise rejection filter, a matched filter, and an equalizer. Although the noise filtering circuit and the imaginary noise rejection circuit are embedded in the QPSK demodulator, the QPSK demodulator may not correctly demodulate data if data reception is poor.
If demodulation is correctly performed, the QPSK demodulator informs a host device of normal demodulation by generating a lock signal. However, because the lock signal is generated using only a mean square error (MSE) value, the lock signal may be unreliable.